Filament forming method



Aug. 1, 1950 s. F. VARIAN FILAMENT FORMING METHOD Filed April 14, 1944INVENTOR 5'. E'VAR/AI/V Patented Aug. 1, 1950 FILAMENT FORMING METHODSigurd F. Varian, Garden City, N. Y., minor to The S ware perryCorporatiomn corporation of Dela- Application April 14, 1944, Serial No.531,100

Claims. (01. 29-2548) This invention is concerned with methods forforming filamentary materials into selected patterns or shapes and isparticularly related to an improved method of producing spirally woundfilaments for electronic apparatus which are capable of maintainingtheir original size, shape and spacing between turns under actualconditions of use, including heating to normal temperatures of emission.

Prior art coils, grids and filaments have been characterized by twomajor defects, namely lack of uniform spacing between turns, anddistortion from the desired size and shape when electric current iscaused to flow through them. These .defects of presently known filamentshave made it extremely difiicult to obtain uniform electron emissionover the face of the filament, and to produce electronic tubes havingthe desired uniformity of performance. a

While the present invention is aimed at overcoming the above describedproblems oi the'electron tube manufacturing art it is also intended tofacilitate the manufacture of precision coils, precision springs andsimilar forms from materials of many kinds where uniformity of spacingand permanence of shape are among the requirements to be met by asatisfactory finished product.

With the previously used methods of manufacturing spiral tungstenfilaments, for example,

it has been customary to wind the filament on a steep conically shapedmold having a special groove on its surface into which the filament isguided as it is wound. The filament is usually heated while still on themold and when removed from the mold naturally retains a conical shape.In an attempt to change the conically shaped coil to a fiat spiral coil,the filament is compressed between two fiat molybdenum plates and firedat high heat to remove the bending stresses from the tungsten and forceit to take a permanently flat set. The major defect of this method isthat the bending stresses in the highly elastic tungsten wire are notentirely removed and the residual stresses which remain within thefilament cause it to resume a semi-conical or v z The abovedescribed-winding method has the further disadvantage that the spacingbetween turns of the filaments loses its uniformity during the processof transferring the coils from the conical mold to the flat molybdenumpressing plates since during this step the filamentary coil .is not heldagainst movement caused either by internal stress, handling. ormisalignment of process equipment. There also exists the furtherdrawback that any desired change in the. spacing between turns of thefilament requiresthe construction of anew forming mold or jig.

If an attempt is made to wind the tungsten wire into a flat spiralprecision coil without properly guiding and supporting the wireduringthe process, the wires which are usually circular in cross sectionwill not hold the proper alignment but will roll and slip over oneanothercausing otherwise distorted shape when installed as the cathodeof an electron tube and heated to normal emission temperatures. In orderto remove all distorting stresses from tungsten, for example, it wouldbe necessary to heat the wire to approximately 1200" C. but when heatedto this high temperature and then allowed to cool it would cease to beflexible, becoming instead very brittle and therefore easilydestructible.

the spiral either to collapse or to become greatly distorted. V

From the point of view 01' the tube manufacturer it is highly desirableto be able to construct a flat coiled filament which will remain free ofdistortion during actual operation, in order to prevent variation intube characteristics resulting from change in shape of the electronsource.

An ideal spirally wound filament complies with the following majorrequirements:

(a) The turns must all lie in the same plane. (b) The spacing betweenturns must be uniform.

(c) The coil must be of the correct size.

(d) It must have a form which-is easy to manufacture.

(e) It must be readily duplicable.

(f) It must be protected against damage durcorrect size: and

The present novel forming method makes posi- 1 sible a high degree ofsuccess in meeting the above requirements in the actual manufacture offilamentary coils and grids of various types.

One object of the invention is to provide an easily performed method ofwinding uniform, ac-

curately sized, electron tube filaments or oath:- odes.

Another and important object of this invention is to provide a method ofmanufacturing coils or other selected shapes from relatively elasticfilamentary and other slender substances such that the residualdistorting stresses remain;-

ing within the material at the end of the process will be as small as itis possible to make them.

It is a further object of this invention to provide a method of bendingfilaments and slender material into fiat shapes in which the residualstresses unavoidably left in the filaments are of such a nature that anyunwinding or movement caused thereby will occur only in the plane of thefiat coil or shape.

It is also an object of this invention to provide a winding or formingmethod for use with filamentary substances which will provide completeprotection of the processed material from nicking or other damage duringthe winding thereof.

A further object of the invention is to provide a method and apparatusfor shaping filamentary materials which is readily adaptable to theproduction of coils or forms of different sizes and shapes.

It is likewise an object of the invention to provide a method ofmanufacture broadly applicable in accurately and permanently bendingelastic filamentary and elongated thin substances of many kinds into awide variety of forms and shapes.

Referring now to the drawing wherein for illustrative purposes themethod of the invention is shown applied to the formation of fiat spiralfilaments:

Fig. 1 is an isometric view of a tungsten or other filament extendingthrough a tube of metal or other readily bendable substance.

Fig. 2 is an isometric view of the tube and filament after the tube hasbeen rolled fiat on two opposite sides.

Fig. 3 is a plan view of the tube and inclosed filament in the processof being formed on a winding jig or forming machine.

Fig. 4 is a cross section of the partly wound coil taken along the line4-4 of Fig. 3.

Fig. 5 is a plan view showing the completely wound coil encased in afusible composition or solder and held in shape by a metallic strip bentand tightly crimped about the coil.

Fig. 6 is a sectional view along line 6-6 of Fi 5.

Fig. 7 is a plan view of the filament in its'final form after annealingand after the surrounding tube and solder have been etched away.

Fig. 8 illustrates an alternative method of holding the coil to size andshape during the annealing and etching steps.

While the invention has been used with particular success in formingtungsten filaments with the aid of medium-hard-drawn copper tubing andwill therefore be described primarily against the background of thisspecific application, it should be understood that the invention isprimarily in a method that may be employed for forming a wide variety ofsubstances into many different shapes and patterns. Flexible plasticmaterials for example, could be shaped to desired patterns by the methodof this invention.

In place of tungsten any other suitable slender and filamentarysubstance, either elastic or non-elastic, may be used provided thesubstance is flexible and can be readily bent. In place of copper, thetube may be composed of other materials, metallic or non-metallic,provided they have sufflcient strength to bend the inclosed filament,are themselves sufficiently flexible to per mit bending into the desiredshapes and will Withstand the annealing temperatures which must be usedwith the chosen filamentary material. The material used for the tubealso 4 a should be such as can be removed from the completed coilwithout damage to the filament itself as by corrosive chemical action orotherwise.

The formation of a flat spiral filament or heater coil from tungstenwire provides a typical illustration of the use of the method of thisinvention. For this purpose the method of the invention is carried outin the following manner.

A piece of copper tubing or conduit of a length determined by the sizeof the filament which it is desired to produce is selected along with a.piece of tungsten wire, the tungsten wire being about an inch longerthan the tubing, and the inside diameter of the tubing preferably beingjust large enough to permit the tungsten wire Next the copper tubing orsheath is rolled or compressed to produce two flat surfaces on oppositesides thereof as shown in Fig. 2, this procedure producing two surfaceswhich are suitable for rolling or winding one upon the other. Obviouslyif square or rectangular copper duct of a size proper to accommodate thetungsten wire is available for use, the rolling process above mentionedmay be dispensed with. To a certain extent the spacing between the turnsof the completed filament can be varied by controlling the extent of therolling to which the copper tubing is subjected, more closely spacedturns being obtained if the copper tubing is rolled relatively thin.Where wider spacing between turns of the cathode is desired than can beobtained by controlling the rolling procedure in the manner described itbecomes necessary to start with tubing having a greater wall thicknessinitially.

If the copper tubing is sufllciently flexible to bend readily it may nowbe placed in a winding jig for forming to the desired shape. However ifthe tubing is too stiff to bend easily it must be annealed beforeforming and this annealing should preferably be done in a hydrogenfurnace to avoid oxidation. Unless the tubing and the inclosed wire canbe bent readily the forming apparatus may not be able to withstand thestrains occasioned by the bending; and breakage or failure of thisapparatus will occur frequently.

Having annealed the copper tube, if necessary, we may now place the tubeand the inclosed filament on a winding or forming jig or mold, shown inFig. 3 as a winding table II having two tube-holding pins 18 and I9,this apparatus being suitable for forming a fiat spiral non-inductivecoil when the copper tubing is wound upon it, fiat side to fiat side, asillustrated. The winding jig is so constructed that the distance betweenpins l8 and [9 may be adjusted to meet the varying space requirements ofcoils having diilerent spacing between turns. As the coil is wound theturns are forced and held tightly against one another by restraining thefree ends of the tubing. The winding is continued until the entire tubeand filament except short portions at the ends suitable for terminalprojections have been brought within the spiral to form a heater elemeatof the desired size. The disposition of the tubing and the tungsten wireat this stage of the process isshown in Fig. 4 where the function of thetubing in controlling the spacing between theannealing, and immersed ina corrosive chemical solution for etching away the sheath.

While the coil is still on the forming Jig molten solder or a similarfusible alloy or composition is deposited on the coil and allowed tosolidify into a temporary protective casing around the same. In Fig.3this protective casing 20 is shown on the right hand portion of thecoil. Thus encased in solder the coil may be removed from the windingjig and immediately placed upon a sheet of nickel or other suitableheat-resistive material which is rigid enough to firmly hold the coilduring subsequent annealing operations. The hickel sheet 2! is then benttightly up around the coil in its final form and shape and the corners2i" are then tightly crimped over the adjacent turns ofthe coil or gridto hold it securely against unwinding and to help protect it from damageby nicking or abrading. The condition of the coil at this stage of theprocess is shown in Figs. 5 and 6. Pure indium has been successfullyused asthe fusible alloy in the above described step of the process andmay be preferable since it does not damage a hydrogen furnace andreadily decomposes w'ith the copper sheath when exposed to nitric acid.

If preferred, the use of solder in the manner above described may beavoided by the adoption of the following alternative procedure. Nickelsheet or strip 2! may be provided with two holes large enough for pinsl8 and IQ of the winding -jifg' to pass through. Strip 2| may thus beplaced flat on the table of winding jig I! with the coil thereafterbeing wound on top of strip 2!, the edges of which may then be crimpedover the coil in the manner previously described upon completion of thewinding and before removing the coil from the winding jig.

In forming a coil from tungsten or other elastic material it becomesnecessary at this stage of the process to subject the coil to a secondand final annealing to the end that bending stresses created in thewinding of the co l and which would result in deformation thereof if noteliminated may be reduced to the lowest practical magnitude beforeremoving the copper tubing and the nickel holding strip. At this pointthen the coil and its supporting strip are placed in a hydrogenannealing furnace and subjected to full annealing temperature for 10minutes or longer. The heat of the furnace naturally disintegrates thesolder or fusible alloy in which the coil is encased during theannealing process. However the nickel strip which was crimped about thecoil in the manner shown in Fig. 5 will continue to hold the coilagainst radial expansion during annealing, even after the solder hasvaporized and the coil has been removed from the annealing furnace.

The final step in carrying out the method of the invention consists ofremoving the copper tube or sheathing and the holding strip whichsurround the tungsten wire, leaving the finished filament free andunencumbered by any supporting structures or substances. One method bywhich this final step may be accomplished is to subject the coil to theaction of some chemical agent, nitric acid for example, which will etchaway or disintegrate the copper tubing and any of the soldering compoundwhich may still be adhering to the coil, without aflecting the tungstenor other filamentary material being shaped. As the copper tubing isdisintegrated by the selectedchemical agent the nickel holding orsupporting strip will become loosened from the coil and may be removedto be set aside for use in forming further coilsby the method of theinvention.

The completed coil in its final form after the removal of the coppertubing and the temporary supporting means is shown in Fig. 7 with theuniform spacing 24' between turns clearly evident.

Fig. 8 shows an alternative method of supporting the coil during theannealing and etching steps of the process. A rigid bar or support, spotwelded or brazed to the copper tubing after formation of the coil, issubstituted for the soldering compound and the nickel crimping stripspecified in the method as originally described. This supporting bar maybe readily removed from the coil after the copper has been etched awayor if preferred the bar itself may be removed by chemical corrosion.

It is of course obvious that if the filamentary material being shaped tosize and pattern is nonelastic, a flexible plastic material for example,

the second annealing step mentioned in the above described procedurewould be unnecessary and could therefore be dispensed with. If themethod of the invention were being employed to produce springs orsimilar resilient forms the second annealing step might also .beeliminated from the forming process.

In forming coiled filaments from elastic materials similar to tungstenit is not desirable to remove all elasticity from the coil by annealingit completely as this would render the coil brittle and unworkable. Oneof the advantages of this invention is that the coil or form is shapedor bent in the same plane in which it is later to be used so thatunwinding or relief of the small residual stresses still remaining inthe coil at the completion of the forming process takes place in theplane of the coil and therefore does not materially affect the spacingbetween cathode and plate of the tube in which the filament isinstalled. In actual practice the method of this invention has been usedto produce filaments having an entirely satisfactory degree ofuniformity.

As many changes could be made in the details of the steps by whieh'themethod of this invention is carried out without departing from the scopethereof, it is intended that all matter contained in the specificationand claims or shown in the accompanying drawing be interpreted in anillustrative and not in a limiting sense.

Having particularly described and illustrated the method of theinvention and its use, what is claimed is as follows:

1. A method of forming uniformly sized precision coils from relativelyhard, springy filamentary material, comprising encasing a flexiblefilament of said material in an inclosing tube formed with a boreclosely fitting the filament for initially straightening the same,flattening said tube on at least two opposite sides, winding said tubeand the, inclosed filament fiat side to flat side upon a supportingsurface to form a coil in which the adjacent turns of the filament areequally and accurately spaced and supported .by the flattened walls ofthe tube, temporarily supporting said coil by encasing it in a fusiblecomposition, surrounding said coil with a stifl binding strip, applyingheat to the tube to anneal the enclosed filament and thereafter removingsaid inclosing tube, binding strip and said fusible composition byimmersion in an etching bath to which said filament is impervious toleave a uniformly spaced coil consisting only of the originalfilamentary material.

2. A forming method as claimed in claim 1 in which indium employed asthe fusible composition.

3. A method of forming hard, resilient fila mentary material accordingto selected patterns having uniform spacing between adjacent elementsthereof, comprising encasing a filament of said material in an inclosingtube, flattening said tube on at least two opposite sides, annealingsaid tube and inclosed filament, forming said tube and inclosed filamentinto the shape of the selected pattern by bending said tube and inclosedfilament fiat side to fiat side upon a supporting surface, temporarilybinding said form by encasing it in a fusible composition, surroundingsaid form with a stiff band, annealing said filament and said inclosingtube, and removing said inclosing tube and said fusible composition byimmersion in a solution corrosive only to said tube, composition andsaid band to leave a uniformly spaced filamentary form composed only ofthe original filamentary material.

4. A method of forming precision-coiled electron tube filaments havinguniformly spaced turns from tungsten wire, comprising thrusting a strandof said wire into a soft metal tube, flattening said tube on at leasttwo opposite sides, winding said tube and the inclosed strand fiat sideto fiat side to form a coil, temporarily supporting said coil byencasing it in a fusible alloy and reinforcing it with a binding strip,annealing said coil and the enclomd tungsten wire, and disintegratingsaid metal tube and said fusible alloy to leave a uniformly spaced flattungsten filament the binding strip being readily detachable after thepartial disintegration of the binding strip.

5. A method of producing uniformly sized tungsten wire coils havinguniformly spaced turns, comprising encasing a filament of tungstenwithin a copper sheath, flattening said sheath on two opposite sides,annealing said sheath in an atmosphere of hydrogen, winding said sheathand the inclosed tungsten filament fiat side to fiat side to form a fiatspiral coil, reinforcing said soil by encasing it in indium, temporarilybinding said coil by crimping a nickel strip around it;

annealing sad filament and said inclosing sheath" in an atmosphere ofhydrogen, and disintegrsting said encasing sheath and said reinforcingindium by exposing said coil to the corrosive' action of nitric acidthus producing a uniformly spaced fiat spiral filament composed only oftungsten the nickel strip being loosened with the disintegration of theencasing sheath, per-' mitting the same to be removed easily during thedisintegration process.

SIGURD F. VARIAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

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